1. Field of the Invention
The present invention relates to a vacuum pump, and more specifically to a vacuum pump having rotor blades arranged on an inlet port side.
2. Description of the Related Art
Vacuum pumps are widely used in, for example, systems for discharging a gas within a chamber and for evacuating the chamber in semiconductor production devices. Such vacuum pumps include those entirely comprised of blades and those comprised of blades and thread groove portions.
FIGS. 6A-6C depict the structures of conventional vacuum pumps. FIG. 6A is a top plan view showing part of a conventional vacuum pump, FIG. 6B is a partial cross-sectional view showing a conventional vacuum pump with a straight inlet port, and FIG. 6C is a partial cross-sectional view showing a conventional vacuum pump with a constricted inlet port.
These vacuum pumps comprise a stator 70 fixed to an interior of a casing 10, and a rotatable rotor 60. The stator 70 and the rotor 60 are formed with axially stepped portions of blades, constituting a turbine.
In vacuum pumps having such a structure, the rotor 60 is rapidly rotated with a motor at several tens of thousand rpm under a normal state, so that the vacuum pumps may be evacuated (exhausted).
Such vacuum pumps are used to discharge gas molecules in such a manner whereby rotation of the rotor 60 allows the gas molecules sucked from an inlet port 16 to be struck in a direction of rotation of rotor blades 62. Depending upon the difference between an amount of the molecules flowing toward the outlet port 17 and the amount of molecules flowing back to the inlet port 16 from the outlet port 17 due to a pressure difference between the inlet port 16 and the outlet port 17, a final discharge amount, i.e., a discharge capability of the pump is determined.
However, the gas molecules within a molecular flow region are reflected in a direction perpendicular with respect to an impinging wall surface (impinging surface) regardless of an angle incident to the wall surface. This urges most of the molecules accelerated in the vicinity of the tip ends of the rotor blades 62 to advance in its tangential direction (a direction vertical to the rotor blades 62). On the other hand, the inner wall of the casing 10 is shaped into a cylinder, and is expanded in a direction of advancing the molecules (tangential direction) depending upon its curvature. Therefore, the gas molecules impinging on the tip ends of the rotor blades 62 may often impinge on the inner wall of the casing 10.
If portions where the rotor blades 62 are arranged have axially constant inner diameters in the casing 10, most of the molecules that accelerate in the vicinity of the tip ends of the rotor blades 62 then impinge on the casing 10, and are reflected in a direction vertical to the wall surface of the casing 10, thereby decelerating in flowing directions. This causes the gas molecules that decelerate in flowing directions (an axial direction) to stay in the vicinity of the tip ends of the rotor blades 62, thereby reducing the discharge flow rate along with a partially increased pressure. This deteriorates discharge capabilities.
This tends to occur at the uppermost rotor blade to which no certain momentum in a discharge direction is yet applied by the rotor blades 62 or in the vicinity of the tip end of the second rotor blade 62 with less momentum.
Consider a turbomolecular pump of the type shown in FIG. 6C, in which the inner diameter of the casing is narrowed at the inlet port side so as to be constricted to a predetermined bore size at the inlet port side (an upstream side) above the uppermost rotor blade 62 in order to attach the casing to a flange with a smaller bore size than the outer diameter of the rotor blades. The gas molecule flow in a molecular flow region is highly straightforward while the gas molecules enter only into substantially the same range as the port size of the inlet port 16. Therefore, the uppermost rotor blade 62 has the problem that the gas molecules are not likely to flow around its tip end (outer peripheral side) with a high flow rate and high discharge efficiency. Hence, the tip end of the uppermost rotor blade 62 is dead space for the gas molecules introduced from the inlet port 16, resulting in less discharging of the gas molecules from the inlet port, and is often used to prevent backflow. The discharging effects are deteriorated.
In order to avoid such disadvantages, it is conceivable that a change ratio of the inner diameter of the constriction of the casing 10 is reduced to increase the gas molecules flowing around the tip end of the uppermost rotor blade 62 from the inlet port. However, an increased distance from the inlet port 16 to the uppermost rotor blade 62 brings less conductance, resulting in no improved discharge rate (effective discharge rate) at the inlet port 16 of the pump.